Recently, absorbent materials have been used in a variety of fields, for example, as materials for paper diapers, sanitary napkins, so-called incontinence pads, for the purpose of absorbing body fluids. Generally, such absorbent materials are produced in the following manner. After powdery or granular absorbent resin is sandwiched between layers of paper, etc., processing operations such as an embossing operation are applied to the paper, etc., or after incorporating absorbent resin with pulp, etc., to form a sheet, film, etc., processing operations such as an embossing operation are applied to the sheet, etc. Instead of the described processing operation, the absorbent resin may be enveloped into a base material using a thermoplastic resin.
A method of manufacturing an absorbent material by forming an absorbent resin into a sheet or a film is disclosed, for example, in Japanese Unexamined Patent Publication No. 141357/1978 (Tokukaisho 53-141357) and U.S. Pat. No. 4,066,583 wherein a mixture of dried powdery absorbent resin and polyvalent alcohol is sandwiched between fluororesin sheets (base materials), and then a pressure is applied thereto. Japanese Unexamined Patent Application No. 174414/1991 (Tokukaihei 3-174414) and U.S. Pat. No. 5,145,906 disclose absorbent materials produced from dried powdery absorbent resin made of polyacrylic acid (salts) and polysaccharides, etc., and paper diapers as an absorbent article.
Furthermore, Japanese Unexamined Patent Publication No. 230671/1989 (Tokukaihei 1-230671) and U.S. Pat. No. 4,826,880 disclose a method of forming hydrate by adding an aqueous solution in an amount of from 20 percent by weight to 80 percent by weight based on a total amount to dried powdery absorbent resin and immobilizing the dried powdery absorbent resin to a base material by extrusion and dispersion. U.S. Pat. No. 5,428,076 discloses a method of immobilizing dried powdery absorbent resin to a base material to be formed into a sheet.
However, according to the described manufacturing method of Japanese Unexamined Patent Publication No. 141357/1978 (Tokukaisho 53-141357), U.S. Pat. No. 4,066,583, Japanese Unexamined Patent Publication No. 174414/1991 (Tokukaihei 3-174414), and U.S. Pat. No. 5,145,906, the water-containing gel resulting from a polymerization reaction is dried, pulverized and classified to form powdery absorbent resin, and the absorbent resin is formed into a sheet or film by incorporating the absorbent resin with the base material. Therefore, the described methods have a problem in that dust generated when forming the absorbent resin into powders makes the handing of the absorbent resin difficult, and working conditions undesirable. Moreover, the yield of the absorbent resin, and the yield of the resulting molded article are lowered. Besides, complicated processes of manufacturing the absorbent material are required, and the absorbent material cannot be manufactured at low cost.
According to the described conventional manufacturing processes, it is required to reduce an amount of use of the absorbent resin particles to be mixed in the fabric matrix in order to prevent possible gel blocking between absorbent resin particles. Therefore, it is difficult to desirably manufacture an absorbent material having a high content of absorbent resin.
Moreover, the absorbent materials resulting from the conventional methods are inferior in their flexibility, strength, etc. Therefore, such absorbent materials are not tolerable to be taken in a roll, etc., or against tension, and it is not possible to manufacture the absorbent materials continuously. Furthermore, when manufacturing sanitary materials (absorbent articles) such as paper diapers, etc., from the absorbent materials, the resulting sanitary materials are not soft and are uncomfortable to skin of the user.
According to the methods of Japanese Unexamined Patent Publication No. 230671/1989 (Tokukaihei 1-230671), U.S. Pat. No. 4,826,880 and U.S. Pat. No. 5,428,076, after once forming the dried powdery absorbent resin, it is formed into a sheet. Materials such as non-woven fabrics are separately required for maintaining the absorbent resin in a sheet form, and a complicated process of incorporating the material into the absorbent resin powders is required. Besides, dust is generated when forming the absorbent resin into a sheet which causes inconvenience in the handling of the absorbent resin. Moreover, it is likely that the resulting absorbent materials are inferior in flexibility of the sheet, absorbing rate, absorbency under load, and shape retaining property of the sheet which makes the weight of the absorbent resin per unit area small, and desirable absorbing properties cannot be obtained.
The present invention is achieved in a hope of finding a solution to the above-mentioned problems, and accordingly, a main object of the present invention is to provide an absorbent resin and an absorbent material which offer excellent absorbing properties such as absorbing rate, absorbency under load, etc., and a shape retaining property. Another object of the present invention is to provide a method of manufacturing the absorbent material which permits the absorbent material to be manufactured at low cost.